The variable nature of fuel products renders them a challenging medium for fluorescence-based analysis. Fuels, depending on fuel type and production conditions, exhibit varying ratios of aromatic and aliphatic components. Moreover, the constituents present in fuel tend to change as the result of oxidative reactions that occur over time. Similarly, variability in fuel compositions arise from the addition of oxygenates (e.g., ethanol, MTBE, and the like) or biologically derived components such as biodiesel.
Changes in fluorescence absorbance and emission bands result from fluctuations in the structure of the solvation shell around a fluorophore. Moreover, spectral shifts (both bathochromic and hypsochromic) in the absorption and emission bands are often induced by a change in solvent mixture or composition; these shifts commonly referred to as solvatochromic shifts, are experimental evidence of changes in the solvation energy. In other words, when a fluorophore is surrounded by solvent molecules, its ground state and excited state are more or less stabilized by fluorophore-solvent interactions, depending on the chemical nature of both the fluorophore and solvent molecules.
Similarly, the fluorescence quantum yield (the ratio of the number of photons emitted to the number of photons absorbed by the fluorophore) is dependent on the solvent in which the analysis is conducted. A variety of non-radiative de-excitation pathways are available and impact the fluorescence efficiency through mechanisms of dynamic or static quenching. Additionally, the temperature of a sample at the time of measurement has an impact on the fluorescence intensity observed for a given quantity of a fluorophore in solution. Generally, an increase in temperature results in a decrease in the fluorescence quantum yield because of an increase in the non-radiative processes related to collisions with solvent molecules, intramolecular vibrations, and rotations.
An additional problem presented when analyzing for a fluorescent taggant in fuels is that of a native variable fluorescence background. Fuels, based on production conditions, chemical composition of starting crude oil, and age of the fuels at the time of analysis, exhibit a natural fluorescence background. This background fluorescence is highly variable and further complicates the quantification of a fluorescent taggant.
An additional problem encountered is the presence of colorants often added to fuels. It is fairly common throughout the world to add visible dyes to fuels; this practice is often employed to allow specific grades or brands of fuel to be visually identified by consumers. The absorption or emission of these dyes can impinge in the spectral response range of a fluorescent taggant, further complicating identification/quantification.
These effects and compositional differences have a dramatic impact on the ability to accurately quantify the amount of a fluorophore present in a fuel of unknown pedigree.